1. Field of the Invention
The present invention relates to a control device for an automotive vehicle, and more particularly to a control device for an automotive vehicle installed with a differential, a differential control device and an antiskid brake system wherein upon failure of the differential control device or the antiskid brake system, a failure signal is sent by the failing device to the non-failing device so that the non-failing device can control the vehicle.
2. Discussion of the Related Art
In general, an automotive vehicle has a differential device to prevent the wheels from slipping even when the difference in wheel speed between right and left wheels is due to a difference between tracks of right and left wheels, to improve vehicle safety.
The differential device is disposed between an engine and the wheels for absorbing a difference in speed between the right and left wheels.
In a front wheel drive vehicle, a front differential device is disposed between the front right and left wheels. In a rear wheel drive vehicle, a rear differential device is disposed between the right and left rear wheels. In a four wheel drive vehicle, in addition to having front and rear differential devices, a center differential device is disposed between the front and rear axles.
A differential limiting device is known in the art to control the power of the differential which restricts the operation of the differential device, and thereby properly transfers the engine output torque to each of the wheels.
Additionally, antiskid brake systems are known in the art which prevent the wheels from locking while the vehicle is braking, and prevent the wheels from slipping. For example, when the wheels suddenly receive a braking force from a brake device of the vehicle, the antiskid brake system reduces the braking pressure on the brake system to reduce the brake force and prevent the wheels from locking just before the wheels are locked, and then it increases the brake pressure again. The antiskid brake system repeats this process until the vehicle stops.
Generally, a vehicle equipped with an antiskid brake system can prevent the wheels from locking and stop in the shortest distance even if the brakes are suddenly applied in icy or snowy driving conditions.
In a vehicle equipped with both an antiskid brake system and a differential limiting device, the front wheels and rear wheels of the vehicle rotate as one and become the same rotational speed if the center differential device locks during the antiskid brake system operation. When this happens, the front and rear wheels mutually interfere so that the antiskid brake system can not detect actual slip conditions of the front and rear wheels. Therefore, it is difficult to independently control the brake pressure of the front and rear wheels corresponding to the slip conditions of both sets of wheels.
Attempts have been made to solve this problem, for example, Japanese Laid-Open Patent Publication 61-287824 discloses that a control device which controls the power of the center differential (of the vehicle equipped with both an antiskid brake system and a differential limiting device) is switched to an un-control condition while the antiskid brake system is operating so that the antiskid brake system can be effective.
In this system, however, when the differential limiting device fails in the locked condition it is not able to be released, and the locked condition will continue while the antiskid brake system is in operation. Thus, there is a possibility that the control device will unnecessarily reduce brake pressure on one or more wheels. Thus, the vehicle may become unstable because all four wheels may lock, or the torque between the driving wheels may become excessive.
Additionally, when the antiskid brake system fails, it is desirable to control the vehicle stability and braking functions by controlling the vehicle differentials.